(1) Field of the Invention
The present invention relates to the field of rotorcraft tail rotors. The present invention relates to an anti-torque device with longitudinal thrust for a rotorcraft. The invention also provides a rotorcraft fitted with such a device. Furthermore, the device is also usable on a drone, i.e. an aircraft not having a human pilot on board. This type of aircraft, generally of size that is small compared with traditional helicopters, is for use in missions of surveillance, information, or combat type.
More particularly, the invention relates to a helicopter having a long range and a high forward speed in cruising flight.
(2) Description of Related Art
A helicopter generally has a single main rotor with a plurality of blades driven mechanically by at least one turbine engine, e.g. the main rotor providing the aircraft with both lift and with propulsion.
A helicopter is traditionally provided with a secondary rotor positioned at the rear end of the aircraft, the secondary rotor providing an anti-torque function in order to compensate for the yaw torque that is created by the rotation of the main rotor, the secondary rotor exerting an opposing transverse thrust. Consequently, such a secondary rotor is referred to as a “tail” rotor or an “anti-torque” rotor by the person skilled in the art. The term tail rotor is used below for convenience.
Furthermore, the tail rotor has a propeller with a plurality of blades, it being possible to modify the pitch of those blades collectively, i.e. to vary the pitch of all of the blades in identical manner. A pilot can thus control yaw and turning movements of the helicopter by modifying the pitch of the blades in the tail rotor so as to modify the magnitude and the direction of the transverse thrust.
The tail rotor is mounted laterally on one end of the helicopter tail boom, or else on a top end of a vertical fin carried by said tail boom.
When installed on a helicopter, the present invention uses longitudinal thrust, making it possible to obtain long range and high forward speed in cruising flight.
Such helicopters relating to an advanced concept of a vertical take-off and landing (VTOL) aircraft are referred to by the person skilled in the art as “hybrid” helicopters.
From the various studies on hybrid helicopters that have already been performed, such an aircraft may comprise the following main elements:
an airframe;
a lift surface secured to the airframe;
stabilization and maneuvering surfaces, namely: in the pitching direction: a horizontal stabilizer with at least one pitching control surface that is movable relative to its front portion or “horizontal plane”; in the yaw direction, at least one suitable stabilizer;
at least one main rotor with the blades of the main rotor having both collective and cyclic pitch control;
at least one propulsive propeller with the blades of the propeller having collective pitch control; and
at least one turbine engine driving the main rotor and the propulsive propeller via a mechanical power transmission.
A first study undertaken by Lockheed® in the context of the Cheyenne® program differs from a conventional helicopter by the presence of two wings of small dimensions in addition to the main rotor and the tail rotor, which wings are for providing lift when flying at high speed, and also a third rotor having three blades that is situated at the end of the tail boom of the aircraft perpendicularly to the conventional tail rotor. The function of the third rotor is to propel the aircraft forwards at high speed.
Subsequently, an experimental Piasecki® SpeedHawk® helicopter has made use of a main rotor having four blades and two short wings on either side of the airframe, the conventional tail rotor being replaced by a ducted tail rotor. The ducted tail rotor performs both the anti-torque function and a longitudinal thrust function by using vectored thrust ducted propeller technology.
A recent study concerning a Sikorsky® X2® aircraft has two main rotors, each with four blades that are superposed above the airframe and that are contrarotating, i.e. they rotate in opposite directions to each other. That technique serves to cancel the turning effect on the airframe generated by a single main rotor and thus makes it possible to eliminate the tail rotor that is conventionally used for opposing yaw torque. However, the aircraft has a third rotor located at the rear end of the airframe perpendicularly to the longitudinal axis of the aircraft in order to add longitudinal thrust.
Because of the presence of a lift portion on either side of the airframe for generating lift in cruising flight, the main rotors can deliver less lift under such conditions, thereby limiting the vibration produced with conventional main rotors. In contrast, the main rotors provide all of the lift of the hybrid helicopter during stages of take-off, landing, and vertical flight.
A recent study undertaken by Eurocopter® presents a different architecture. According to patent FR 2 916 418, the aircraft has a main rotor with five blades and two propellers installed on either side of the airframe of the aircraft on wings of small span. Those two propellers provide the longitudinal thrust needed to enable the helicopter to advance at high speed. In that configuration also, the tail rotor is omitted and the turning effect generated by the main rotor is compensated by the two side propellers, by acting on their respective thrusts.
Once more, the lift from the main rotor can be reduced above a certain longitudinal speed at which the lift portion provides sufficient additional lift, thereby making it possible to limit the amount of vibration generated in comparison with conventional main rotors.
Also known from document JP 2009/051465 is a tail rotor system made up of two ducted propellers located on either side of a vertical stabilizer. The axes of the two propellers form a V-shape in a horizontal plane, with the tip of the V-shape pointing rearwards relative to the aircraft so that the axes of the two propellers splay apart from the tail boom of the helicopter on going towards the front of the aircraft.
The air streams generated by the two propellers act via the vertical stabilizer to oppose the turning effect generated by the main rotor and they also enable the aircraft to be controlled in yaw. Furthermore, those air streams provide longitudinal thrust for propelling cruising flight of the aircraft.
The function of the vertical stabilizer is to stabilize the aircraft by channeling and steering the air streams generated by the propellers, both during hovering flight and during cruising flight. A rudder on the rear portion of the vertical stabilizer serves to accentuate this steering of the streams, e.g. in order to perform yaw maneuvers.
In the same manner, a horizontal stabilizer enables the aircraft to be stabilized and controlled in pitching. The vertical stabilizer between the two rotors also serves to avoid air stream turbulences between propellers by isolating the air streams from each of the propellers.
In contrast, steering the air streams in this way involves a significant loss of aerodynamic efficiency, in particular as a result of friction between the air streams and the stabilizers located behind the propellers.
Document U.S. Pat. No. 3,155,341 describes a convertible helicopter that is capable both of behaving like a helicopter, i.e. that is capable in particular of vertical take-off and landing, and also of behaving like an airplane, in particular during high speed cruising flight. For this purpose, that aircraft has firstly two wings, with horizontal and vertical tail stabilizers like an airplane, and secondly a main rotor with a tail rotor like a helicopter. The tail rotor has the special feature of being capable of being swung as a whole about a vertical axis. It can thus act as a conventional tail rotor in helicopter mode, providing essentially all of the anti-torque function, and it can perform the propulsion function of a propeller in airplane mode by being placed perpendicularly to the longitudinal axis of the aircraft.
Document U.S. Pat. No. 2,698,147 describes a rotary wing aircraft having a main rotor for providing lift and propulsion, and two propellers situated at the rear of the aircraft. A first propeller is positioned perpendicularly to the longitudinal axis of the aircraft and performs the anti-torque function and yaw control of the aircraft, while a second propeller is positioned on the longitudinal axis of the aircraft and provides part of its propulsion.
Document US 2009/0159740 describes a rotary wing aircraft having two main rotors that are coaxial, serving mainly to provide lift, and two ducted propellers situated at the rear of the aircraft to provide it with propulsion and with yaw control. The axes of those two propellers are parallel to the longitudinal axis of the aircraft.